SCT focus problem with focal reducer

Hi Rod -
Thanks for the reply. From what you are saying, it sounds like I would
not have this problem using a 2" eyepiece without the 1.25" adapter. Am I
correct?

"Rod Mollise" wrote in message
...
Has anyone run into this or have any solution? I would really like
to be able to use the William Optics diagonal all the time. Thanks for
any
advice.

Hi:

The problem is the 1.25" adaptor. The solution? A couple of outfits have
sold
"low profile" 1.25"/2" adaptors in the past, notably AstroSystems...that
may
give you enough infocus to cure your problem.

Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html

"DWilson" wrote in message
...
Hi Rod -
Thanks for the reply. From what you are saying, it sounds like I would
not have this problem using a 2" eyepiece without the 1.25" adapter. Am I
correct?
The 'key', is in a lot of old threads about focal reducers. The f*.63
reducer, is a lens, with a focal length of about 240mm. The reduction this
gives, is dependant on the spacing between it and the focal plane of the
eyepiece/film. The reducer gives it's 'design' reduction, when spaced with
about 90mm from the front of the threads, to the focal plane. Spaced like
this, the backfocus required, becomes 90/0.63 + 12mm (12mm, is about the
length of the reducer internally). Hence around 155mm. However if you
increase the reducer spacing, even a little, things start to 'shoot up'. The
problem is that the increased spacing, gives more compression, and the
backfocus rises massively. So (for instance), if the spacing goes up to
110mm, the new compression ratio becomes (240-110)/240 = 0.54*, and the
backfocus becomes 110/0.54 + 12 = 216mm.
You can see that a tiny increase in spacing (20mm), has had a massive effect
on the scope backfocus required (61mm).
Now the use of a 2" eyepiece, or a very 'slim' 2" to 1.25" adapter, will
bring the focal plane, close to the top of the the eyepiece hole in the
larger diagonal, and have a very significant effect, and should reduce the
backfocus to the point where your system will work. Try a very crude
experiment. Focus your scope as far as it will go, and hold a low
magnification eyepiece in the 2" opening. Move the eyepiece up and down till
the image is close to focus. If the top to the eyepiece 1.25" tube is above
the rim of the diagonal tube, then a 2" eyepiece should work fine, and if
there is a reasonable gap, a 'low profile' adapter should also work.

Best Wishes

"Rod Mollise" wrote in message
...
Has anyone run into this or have any solution? I would really like
to be able to use the William Optics diagonal all the time. Thanks for
any
advice.

Hi:

The problem is the 1.25" adaptor. The solution? A couple of outfits have
sold
"low profile" 1.25"/2" adaptors in the past, notably AstroSystems...that
may
give you enough infocus to cure your problem.

Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html

Thanks for the reply. From what you are saying, it sounds like I would
not have this problem using a 2" eyepiece without the 1.25" adapter. Am I
correct?

Hi:

Yes. Oh, you might run into an eyepiece with an extreme focus position, but
normally things should be hunky dory with 2"ers.

Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html

That's why I wish Celestron would make their reducer corrector in 2" filter
format, like the Starsweeper ...

"Roger Hamlett" wrote in message
...

"DWilson" wrote in message
...
Hi Rod -
Thanks for the reply. From what you are saying, it sounds like I
would
not have this problem using a 2" eyepiece without the 1.25" adapter. Am
I
correct?
The 'key', is in a lot of old threads about focal reducers. The f*.63
reducer, is a lens, with a focal length of about 240mm. The reduction this
gives, is dependant on the spacing between it and the focal plane of the
eyepiece/film. The reducer gives it's 'design' reduction, when spaced with
about 90mm from the front of the threads, to the focal plane. Spaced like
this, the backfocus required, becomes 90/0.63 + 12mm (12mm, is about the
length of the reducer internally). Hence around 155mm. However if you
increase the reducer spacing, even a little, things start to 'shoot up'.
The
problem is that the increased spacing, gives more compression, and the
backfocus rises massively. So (for instance), if the spacing goes up to
110mm, the new compression ratio becomes (240-110)/240 = 0.54*, and the
backfocus becomes 110/0.54 + 12 = 216mm.
You can see that a tiny increase in spacing (20mm), has had a massive
effect
on the scope backfocus required (61mm).
Now the use of a 2" eyepiece, or a very 'slim' 2" to 1.25" adapter, will
bring the focal plane, close to the top of the the eyepiece hole in the
larger diagonal, and have a very significant effect, and should reduce the
backfocus to the point where your system will work. Try a very crude
experiment. Focus your scope as far as it will go, and hold a low
magnification eyepiece in the 2" opening. Move the eyepiece up and down
till
the image is close to focus. If the top to the eyepiece 1.25" tube is
above
the rim of the diagonal tube, then a 2" eyepiece should work fine, and if
there is a reasonable gap, a 'low profile' adapter should also work.

Best Wishes

"Rod Mollise" wrote in message
...
Has anyone run into this or have any solution? I would really like
to be able to use the William Optics diagonal all the time. Thanks
for
any
advice.

Hi:

The problem is the 1.25" adaptor. The solution? A couple of outfits
have
sold
"low profile" 1.25"/2" adaptors in the past, notably
AstroSystems...that
may
give you enough infocus to cure your problem.

Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html

What everyone else has said so far about the 2" to 1.25" adapter is right on
target, but there's another thing to consider, as well, the 2" adapter that the
diagonal fits into. I've noticed that my own 2" adapter (an Astrophysics one)
is slightly longer than it needs to be-- that is, the length it adds is
slightly more than the length of the tube of the diagonal that fits into it.
If your diagonal is of the "refractor" type rather than the "SCT" type (which
screws directly into the SCT threads (or focal reducer threads, if one's being
used)), then you might look into this. I believe that TeleVue makes an SCT
adapter that is slightly shorter that could help. I got one of the
Astrosystems 2" to 125" adapters and it helped to bring some of the 1.25"
eyepieces to focus with the reducer/corrector in place, but not all, so one of
my next things will be to try the shorter 2" SCT adapter from TeleVue.

What everyone else has said so far about the 2" to 1.25" adapter is right on
target, but there's another thing to consider, as well, the 2" adapter that
the
diagonal fits into

Hi:

I've seen a couple of _slightly_ shorter adapters, but it seems to me somebody
could make one shorter still. At any rate, I got tired of this setup in the C11
and now use an Eyeopener on the big back. Can't use an f/6.3 reducer easily in
this setup, but I'm testing the Denkmeier Star Sweeper. So far, it is
delivering very good images and I think removing and replacing it on the end of
the diagonal is easier than screwing an r/c on and off a rear port--in spades.

Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html

"Bettrel" wrote in message
...
What everyone else has said so far about the 2" to 1.25" adapter is right
on
target, but there's another thing to consider, as well, the 2" adapter
that the
diagonal fits into. I've noticed that my own 2" adapter (an Astrophysics
one)
is slightly longer than it needs to be-- that is, the length it adds is
slightly more than the length of the tube of the diagonal that fits into
it.
If your diagonal is of the "refractor" type rather than the "SCT" type
(which
screws directly into the SCT threads (or focal reducer threads, if one's
being
used)), then you might look into this. I believe that TeleVue makes an
SCT
adapter that is slightly shorter that could help. I got one of the
Astrosystems 2" to 125" adapters and it helped to bring some of the 1.25"
eyepieces to focus with the reducer/corrector in place, but not all, so
one of
my next things will be to try the shorter 2" SCT adapter from TeleVue.
Yes.
Keeping everything as short as possible, is a useful 'skill', especially
when the optical path length starts to become silly, and a reduction at this
end is definately something that should also be considered.

Best Wishes